It is possible to cause the regression of a 4-day established tumor in a gamma-irradiated recipient by infusing the recipient with one organ equivalent of splenic T cells from a donor bearing a 9-day, progressive lethal tumor. This finding provides a powerful adoptive immunization assay to follow the generation of concomitant antitumor immunity to progressive tumor growth. Experiments with the SA1 sarcoma, Meth A fibrosarcoma, P815 mastocytoma, P388 lymphoma and L5178Y lymphoma revealed that progressive tumor growth evoked the generation of tumor-sensitized T cells that, on passive transfer, could cause the regression of an established tumor in a gamma-irradiated (500 rad) recipient. These T cells were first detected on day 6 of tumor growth, reached peak numbers on days 9 to 10, and were then progressively lost as the tumor increased in size. Only splenic T cells harvested on days 9 to 10 had the capacity to cause complete and permanent regression of the recipient's tumor. The generation and loss of T cells capable of adoptively immunizing against an established tumor corresponded temporally to the acquisition and loss of concomitant immunity to growth of a tumor cell implant. It was next shown, with the Meth A fibrosarcoma and P815 mastocytoma, that the progressive loss after day 9 of T cells capable of adoptively immunizing against an established tumor was associated with the progressive acquisition of splenic suppressor T cells capable of inhibiting the capacity of tumor-sensitized T cells from preimmunized donors to cause the regression of an established tumor in T cell-deficient (TXB) recipients. These findings are consistent with the hypothesis that there is progressive growth of a concomitant immune response which is actively down-regulated by suppressor T cells before it reaches sufficient magnitude to destroy the tumor. The adoptive immunization assay showed that the T cells that mediate concomitant immunity are Ly1-2+ T cells. In contrast, the cells which suppress the immunity are Ly1+2- T cells. Additional experiments have revealed that suppressor T cells function in the adoptive immunization assay by inhibiting the ability of passively transferred helper or memory T cells to give rise to an adoptive cytolytic T-cell response in the tumor-bearing recipient. A knowledge of the kinetics of the generation and T cell-mediated suppression of the concomitant antitumor immune response is crucial for the design of successful immunotherapeutic modalities. The experimental results indicate that any attempt to treat a tumor by active or passive immunotherapy represents an attempt to superimpose an immune response on an ongoing concomitant immune response that may be undergoing negative regulation.